Porous glass produced utilizing the phase-separation phenomenon of glass should be industrially used for optical members having various optical functions, such as antireflection.
In general, a porous glass utilizing the phase-separation phenomenon of glass is produced by subjecting a borosilicate glass having a composition that can be phase-separated to heat treatment at 500 to 700 degrees (Celsius) to cause phase separation and performing etching to form pores. PTL 1 discloses that a porous surface layer of glass is formed by controlling etching conditions, providing an antireflective surface layer. NPL 1 discloses that the control of detailed etching conditions results in a porous layer having a gradient-index structure determined from reflectance characteristics of the porous layer formed on a surface of glass. PTL 2 discloses a porous glass film having a two-layer structure, the two layers having different average pore sizes. The porous glass film in which the pore size of an upper layer is smaller than that of a lower layer is produced by stacking glass layers having different phase-separation rates, forming the stack into a shape, firing the stack, and etching the stack with an acid. PTL 3 discloses a porous antireflection film formed by etching a film composed of two or more substances.
It is known that the formation of a subwavelength structure is commonly used as a method for achieving good antireflection performance. For example, in the case where an ideal film having a subwavelength structure is formed on a base (letting the refractive index of the base be equal to that of the film), suppose that the film is divided into layers, the space occupancy of the layers is continuously changed from 0% to 100% at greater distances from air toward the base. The effective refractive index is continuously changed from the refractive index of air to the refractive index of the base. This significantly reduces reflection at interfaces between the layers, thereby achieving antireflection performance having good wavelength-band characteristics and incident-angle characteristics.
PTL 1 and NPL 1 each report that the porous surface layer of glass is formed to provide the antireflective surface layer. However, the formation of the antireflective surface layer depends on an etching process of a phase-separated leached phase. So, the formation of the layer is limited to the etching conditions. The conditions of the etching process which can be selected are narrow. In addition, it is difficult to strictly control the etching process. Accordingly, the control range of the layer formation is limited, and the degree of flexibility in the resulting gradient-index structure is low. So, the subwavelength structure is not sufficient. It is thus difficult to achieve excellent antireflection performance.
In the case of the porous glass film having the two-layer structure disclosed in PTL 2, the two layers having different average pore sizes, the structure is not continuous; hence, the porous glass film is not suitable as an antireflective component. In the case of the porous antireflection film formed by the selective etching of a film composed of two or more substances disclosed in PTL 3, the resulting porous structure significantly depends on the etching conditions as described above. This limits the control of the structure. Thus, the porous antireflection film is not satisfactory as an antireflective component.
As described above, in the related art, the formation of an antireflective layer significantly depends on an etching process, thus leading to a low degree of flexibility in the formation of an excellent antireflective structure. There is no literature on the production of a porous glass having excellent antireflection performance by widely controlling a porous structure.